A known image forming apparatus includes a transfer unit, e.g., belt unit. The belt unit includes a pair of rollers, a frame holding the rollers, and an endless belt extending around the rollers. Generally, the belt unit is detachably attached to an apparatus body, e.g. a main body frame, of the image forming apparatus.
In the image forming apparatus, a locking mechanism is provided which engages a supporting portion that protrudes inward from a side of the apparatus body and a supported portion that protrudes in a width direction of the belt unit. The belt unit is fixed to the apparatus body by the locking mechanism. Thus, the locking mechanism, the supported portion and the supporting portion make the outer dimensions of the belt unit increased especially in the width direction.
If the size of the locking mechanism is decreased, the outer dimensions of the belt unit may be prevented from increasing, but a fixing force to fix the belt unit to the apparatus body may become undesirably small. Thus, when the image forming apparatus is moved, the belt unit may be accidentally moved out of position with respect to the apparatus body due to an excitation force applied to the image forming apparatus.